Generally, in mobile communication devices based on systems such as GSM (Global System for Mobile Communication), EDGE (Enhanced Data Rates for GSM Evolution), DECT (Digital Enhanced (formerly European) Cordless Telecommunications), etc., the output power of transmitters is adjusted to reduce power consumption and minimize interference with other signals. The adjustments in the output power are usually performance in accordance with pre-defined standards that govern the implementation of such systems. For example, GSM specifications list nominal output power levels, and accepted tolerances of GSM mobile transmitters under various operating conditions. In existing systems, the output power is adjusted by regulating either the operating voltage or the quiescent current in a power amplifier.
Control of output power by regulating the operating voltage depends on the load impedance of the power amplifier. The load impedance should be maintained at a constant value for effective power control; however, in general, the load impedance may be susceptible to variations. For example, if a cell phone is placed on a metallic object, the antenna of the cell phone may pick up some noise, causing variations in load impedance of the power amplifier inside the cell phone. Power control rising voltage regulation is typically most effective when the power amplifier operates in the saturation region. In some systems, such as GSM and DECT systems, the power amplifier operates in the saturation region, while in other systems, such as EDGE, 3G (WCDMA), Wireless LAN, the power amplifier operates in the linear region. Therefore, power control by voltage regulation cannot be used effectively in all systems.
Output power control by regulating quiescent current involves adjusting the quiescent current to change the output power. This approach works well for small outputs since the current consumption is low; however, there remain issues related to transistor parameters, temperature, and dependence on input power.